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Crevice corrosion (CC) of metals remains a serious concern for structural materials. Yet a real-time in situ visualization of corrosion, and its inhibition within a confined geometry, remains challenging. Here, we present how multiple-beam interferometry in a Surface Forces Apparatus can be utilized to directly visualize corrosion processes in real-time and with Angstrom resolution within welldefined confinement geometries. We use atomically smooth muscovite mica surfaces to form round-shaped ∼1000 μm2 crevices on aluminum. After exposure to NaCl solutions we can detect and track active sites of aluminum corrosion within this confined geometry. CC of aluminum randomly initiates in the confined crevice mouth, where the distance between apposing surfaces is between 20-300 nm. We can directly track oxide dissolution/formation, and corrosion-rates as well as their retardation due to sodium vanadate inhibitors present in solution. Formation of a compacted oxide layer effectively inhibits CC in 5 mM NaCl solutions with 2.5 mM of added NaVO3, while inhibition rapidly breaks down at chloride concentrations above 50 mM. Breakdown of the inhibition-layers is initiated by rapid dissolution of the protective oxide within the confined zone. Our technique may be adapted for monitoring CC, corrosion inside of crack-tips, and evaluation of inhibitor efficiencies in a variety of metals.

Polymers arewidely utilized as protective coatings to prevent metal surfaces from wear, corrosion, or bio-fouling.Yet, stability and adhesive properties of polymer to metal bondings are not fully understood at themolecular level. Here,wemeasured the interaction forces between a gold electrode surface and amine-functionalized polymers (PEG) using a novel electrochemical surface forces apparatus. We examined the potential dependence of specific amine.gold interactions and measured that the binding strength of amine.gold bonds can range from 0.5 to 40 kBT per binding site, depending on the applied electrochemical potential. Notably, this interaction exhibits a pronounced minimum around the potential of zero charge, where the polymer.gold adhesion is dominated by non-specific interactions between the polymer backbone and electrode surface, consistent with the adhesion of PEG polymers with gold surfaces in the absence of amine functionalization. Further, repulsive hydration interactions were observed to be stronger for amine-terminated PEG compared to non-functionalized PEG, due to increased hydration and presence of counterions in the case of amine-terminated PEG. Our results provide molecular-scale insight into design and optimization of polymer coatings for numerous applications requiring strong, specific binding interactions between polymers and metals (or their native oxides), for example passivating layers in biomedical implants and electronic devices. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Controlling polymer-metal adhesion is critical in ensuring that materials can be cleanly separated during production processes without residue, which is crucial for various industrial applications. Accurately characterizing adhesion on industrial-grade surfaces is complex due to factors like surface roughness and actual contact area between surfaces and the polymer. In this study, we quantified the adhesive behavior of stainless-steel samples with varying surface treatments against a polymer using the surface forces apparatus (SFA) in reflection geometry, as well as X-ray photoelectron spectroscopy (XPS). We compared adhesive properties with the penetration depth of oxygen and the hydroxide-to-oxide ratio, which were modified by plasma and thermal treatments. Our results indicate that both treatment types enhance the deadhesive properties of the materials compared to native passive films, due to decreasing hydroxide functionality on the surface. Thermal treatment reduces adhesion further, due to an even lower hydroxide content, which reduces hydrogen bonding between the surface and polymer. Furthermore, we show that van der Waals forces, which depend on the density, have marginal to no influence on the adhesive behavior. This study not only advances our understanding of the factors influencing polymer-metal adhesion but also demonstrates the application of the SFA in reflection geometry for characterizing industrially relevant rough surfaces.

Multi-element alloys and high-entropy alloys show promising electrocatalytic behavior for water splitting and other catalytic reactions, due to their highly tunable composition. While preparation and synthesis of these materials are thoroughly investigated, the true reactive surface composition is still not well understood, as it may significantly differ from the bulk composition. Precise knowledge and understanding of resulting surface composition is crucial for effective control of the electrocatalytic performance. In this work, low energy ion scattering spectroscopy was applied to determine the surface oxide composition of a series of Ni-based multi-metallic alloys with Mn, Fe, Co, and Cr under alkaline, neutral and acidic conditions. The composition of the surface oxide was investigated with sub-nanometer depth resolution. In electrochemical tests, good catalytic activity was found for the oxygen evolution reaction, although a strong dependence on the selected reaction conditions was observed. The surface composition under OER conditions deviates significantly from the bulk composition. No significant benefit of high entropy alloying compared with binary or ternary alloys concerning catalytic OER performance was found., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Carbon-based electrodes are used in flow batteries to provide active centers for vanadium redox reactions. However, strong controversy exists about the exact origin of these centers. This study systematically explores the influence of structural and functional groups on the vanadium redox reactions at carbon surfaces. Pyridine, phenol and butyl containing groups are attached to carbon felt electrodes. To establish a unique comparison between the model and real-world behavior, both non-activated and commercially used thermally activated felts serve as a substrate. Results reveal enhanced half-cell performance in non-activated felt with introduced hydrophilic functionalities. However, this cannot be transferred to the thermally activated felt. Beyond a decrease in electrochemical activity, a reduced long-term stability can be observed. This work indicates that thermal treatment generates active sites that surpass the effect of functional groups and are even impeded by their introduction., (© 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

The degradation and aging of carbon felt electrodes is a main reason for the performance loss of Vanadium Redox Flow Batteries over extended operation time. In this study, the chemical mechanisms for carbon electrode degradation are investigated and distinct differences in the degradation mechanisms on positive and negative electrodes have been revealed. A combination of surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Electrochemical Impedance Spectroscopy (EIS) was applied for this purpose. In addition to understanding the chemical and physical alterations of the aged electrodes, a thermal method for reactivating aged electrodes was developed. The reactivation process was successfully applied on artificially aged electrodes as well as on electrodes from a real-world industrial vanadium redox flow battery system. The aforementioned analysis methods provided insight and understanding into the chemical mechanisms of the reactivation procedure. By applying the reactivation method, the lifetime of vanadium redox flow batteries can be significantly extended.

Ion interactions with interfaces and transport in confined spaces, where electric double layers overlap, are essential in many areas, ranging from crevice corrosion to understanding and creating nano-fluidic devices at the sub 10 nm scale. Tracking the spatial and temporal evolution of ion exchange, as well as local surface potentials, in such extreme confinement situations is both experimentally and theoretically challenging. Here, we track in real-time the transport processes of ionic species (LiClO 4 ) confined between a negatively charged mica surface and an electrochemically modulated gold surface using a high-speed in situ sensing Surface Forces Apparatus. With millisecond temporal and sub-micrometer spatial resolution we capture the force and distance equilibration of ions in the confinement of D ≈ 2-3 nm in an overlapping electric double layer (EDL) during ion exchange. Our data indicate that an equilibrated ion concentration front progresses with a velocity of 100-200 μm s -1 into a confined nano-slit. This is in the same order of magnitude and in agreement with continuum estimates from diffusive mass transport calculations. We also compare the ion structuring using high resolution imaging, molecular dynamics simulations, and calculations based on a continuum model for the EDL. With this data we can predict the amount of ion exchange, as well as the force between the two surfaces due to overlapping EDLs, and critically discuss experimental and theoretical limitations and possibilities.

Membrane proteins are among the most difficult to study as they are embedded in the cellular membrane, a complex and fragile environment with limited experimental accessibility. To study membrane proteins outside of these environments, model systems are required that replicate the fundamental properties of the cellular membrane without its complexity. We show here a self-assembled lipid bilayer nanoarchitecture on a solid support that is stable for several days at room temperature and allows the measurement of insect olfactory receptors at the single-channel level. Using an odorant binding protein, we capture airborne ligands and transfer them to an olfactory receptor from Drosophila melanogaster (OR22a) complex embedded in the lipid membrane, reproducing the complete olfaction process in which a ligand is captured from air and transported across an aqueous reservoir by an odorant binding protein and finally triggers a ligand-gated ion channel embedded in a lipid bilayer, providing direct evidence for ligand capture and olfactory receptor triggering facilitated by odorant binding proteins. This model system presents a significantly more user-friendly and robust platform to exploit the extraordinary sensitivity of insect olfaction for biosensing. At the same time, the platform offers a new opportunity for label-free studies of the olfactory signaling pathways of insects, which still have many unanswered questions.

Water-based lubricants provide lubrication of rubbing surfaces in many technical, biological, and physiological applications. The structure of hydrated ion layers adsorbed on solid surfaces that determine the lubricating properties of aqueous lubricants is thought to be invariable in hydration lubrication. However, we prove that the ion surface coverage dictates the roughness of the hydration layer and its lubricating properties, especially under subnanometer confinement. We characterize different hydration layer structures on surfaces lubricated by aqueous trivalent electrolytes. Two superlubrication regimes are observed with friction coefficients of 10 -4 and 10 -3 , depending on the structure and thickness of the hydration layer. Each regime exhibits a distinct energy dissipation pathway and a different dependence to the hydration layer structure. Our analysis supports the idea of an intimate relationship between the dynamic structure of a boundary lubricant film and its tribological properties and offers a framework to study such relationship at the molecular level.

The aeronautical industry demands facile lightweight and low-cost solutions to address climate crisis challenges. Graphene can be a valid candidate to tackle these functionalities, although its upscalability remains difficult to achieve. Consequently, graphene-related materials (GRM) are gathering massive attention as top-down graphite exfoliation processes at the industrial scale are feasible and often employed. In this work, environmentally friendly produced partially oxidized graphene nanosheets (POGNs) reduced by green solvents such as l-Ascorbic Acid to rGNs are proposed to deliver functional coatings based on a glass fiber composite or coated Al2024 T3 for strategic R&D questions in the aeronautical industry, i.e. , low energy production, de-icing, and water uptake. In detail, energy efficiency in rGNs production is assessed via response-surface modeling of the powder conductivity, hence proposing an optimized reduction window. De-Icing functionality is verified by measuring the stable electrothermal property of an rGNs based composite over 24 h, and water uptake is elucidated by evaluating electrochemical and corrosion properties. Moreover, a mathematical model is proposed to depict the relation between the layers' sheet resistance and applied rGNs mass per area, which extends the system to other graphene-related materials, conductive two-dimensional materials, and various substrates. To conclude, the proposed system based on rGNs and epoxy paves the way for future multifunctional coatings, able to enhance the resistance of surfaces, such as airplane wings, in a flight harsh environment.

The plasma membrane protects the interiors of cells from their surroundings and also plays a critical role in communication, sensing, and nutrient import. As a result, the cell membrane and its constituents are among the most important drug targets. Studying the cell membrane and the processes it facilitates is therefore crucial, but it is a highly complex environment that is difficult to access experimentally. Various model membrane systems have been developed to provide an environment in which membrane proteins can be studied in isolation. Among them, tethered bilayer lipid membranes (tBLMs) are a promising model system providing a solvent-free membrane environment which can be prepared by self-assembly, is resistant to mechanical disturbances and has a high electrical resistance. tBLMs are therefore uniquely suitable to study ion channels and charge transport processes. However, ion channels are often large, complex, multimeric structures and their function requires a particular lipid environment. In this paper, we show that SthK, a bacterial cyclic nucleotide gated (CNG) ion channel that is strongly dependent on the surrounding lipid composition, functions normally when embedded into a sparsely tethered lipid bilayer. As SthK has been very well characterized in terms of structure and function, it is well-suited to demonstrate the utility of tethered membrane systems. A model membrane system suitable for studying CNG ion channels would be useful, as this type of ion channel performs a wide range of physiological functions in bacteria, plants, and mammals and is therefore of fundamental scientific interest as well as being highly relevant to medicine.

An airplane is statistically struck by lightning every year. The need for lightweight aircraft to reduce the production of carbon dioxide has significantly reduced the presence of metals in favour of composites, resulting in lower lightning strike protection efficiency. In this perspective, we critically review the state of technologies in lightning strike protection solutions based on carbon materials, graphene, and MXenes. Furthermore, we comment on possible future research directions in the field.

Muscovite mica, KAl 2 (Si 3 Al)O 10 (OH) 2 , is a common layered phyllosilicate with perfect cleavage planes. The atomically flat surfaces obtained through cleaving lend themselves to scanning probe techniques with atomic resolution and are ideal to model minerals and clays. Despite the importance of the cleaved mica surfaces, several questions remain unresolved. It is established that K + ions decorate the cleaved surface, but their intrinsic ordering - unaffected by the interaction with the environment - is not known. This work presents clear images of the K + distribution of cleaved mica obtained with low-temperature non-contact atomic force microscopy (AFM) under ultra-high vacuum (UHV) conditions. The data unveil the presence of short-range ordering, contrasting previous assumptions of random or fully ordered distributions. Density functional theory (DFT) calculations and Monte Carlo simulations show that the substitutional subsurface Al 3+ ions have an important role for the surface K + ion arrangement., (© 2023. The Author(s).)

Osteoarthritis is the most common degenerative and highly prevalent joint disease, characterized by progressive loss and destruction of articular cartilage. The damaged cartilage surface has an increased friction, which causes patients to suffer from serious pain. Restoring the lubrication ability of the joint is central to the treatment of osteoarthritis, a key topic in medical research. A variety of lubricants have been designed to reduce friction in joints and promote cartilage tissue repair to alleviate the symptoms of osteoarthritis. Herein, we review the recent progress of lubricants from the three perspectives of natural, bioinspired, and alternative strategies for osteoarthritis treatment, as well as the structural characterization and lubrication properties of such lubricants. Specifically, natural lubricants include glycosaminoglycans, lubricin and lipids in joints, bioinspired lubricants include scaffolds mimicking hyaluronic acid or lubricin, and alternative lubricants include modified lubricants based on hyaluronic acid, lipids, nanoparticles, and peptides. We also discuss the current challenges and long-term perspectives for further research in this area., Competing Interests: Declaration of Competing Interest The authors declare that they have no conflict of interest., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The light-driven reactions of photosynthesis as well as the mitochondrial power supply are located in specialized membranes containing a high fraction of redox-active lipids. In-plane charge transfer along such cell membranes is currently thought to be facilitated by the diffusion of redox lipids and proteins. Using a membrane on-a-chip setup, we show here that redox-active model membranes can sustain surprisingly high currents (mA) in-plane at distances of 25 μm. We also show the same phenomenon in free-standing monolayers at the air-water interface once the film is compressed such that the distance between redox centers is below 1 nm. Our data suggest that charge transfer within cell walls hosting electron transfer chains could be enabled by the coupling of redox-lipids via simultaneous electron and proton in-plane hopping, similar to conductive polymers. This has major implications for our understanding of the role of lipid membranes, suggesting that Q-lipid-containing membranes may be essential for evolving the complex redox machineries of life., Competing Interests: The Authors declare no competing Interests., (© 2022 The Authors.)

Extremely robust cohesion triggered by calcium silicate hydrate (C-S-H) precipitation during cement hardening makes concrete one of the most commonly used man-made materials. Here, in this proof-of-concept study, we seek an additional nanoscale understanding of early-stage cohesive forces acting between hydrating model tricalcium silicate (C 3 S) surfaces by combining rheological and surface force measurements. We first used time-resolved small oscillatory rheology measurements (SAOSs) to characterize the early-stage evolution of the cohesive properties of a C 3 S paste and a C-S-H gel. SAOS revealed the reactive and viscoelastic nature of C 3 S pastes, in contrast with the nonreactive but still viscoelastic nature of the C-S-H gel, which proves a temporal variation in the cohesion during microstructural physicochemical rearrangements in the C 3 S paste. We further prepared thin films of C 3 S by plasma laser deposition (PLD) and demonstrated that these films are suitable for force measurements in the surface force apparatus (SFA). We measured surface forces acting between two thin C 3 S films exposed to water and subsequent in situ calcium silicate hydrate precipitation. With the SFA and SFA-coupled interferometric measurements, we resolved that C 3 S surface reprecipitation in water was associated with both increasing film thickness and progressively stronger adhesion (pull-off force). The lasting adhesion developing between the growing surfaces depended on the applied load, pull-off rate, and time in contact. These properties indicated the viscoelastic character of the soft, gel-like reprecipitated layer, pointing to the formation of C-S-H. Our findings confirm the strong cohesive properties of hydrated calcium silicate surfaces that, based on our preliminary SFA measurements, are attributed to sharp changes in the surface microstructure. In contact with water, the brittle and rough C 3 S surfaces with little contact area weather into soft, gel-like C-S-H nanoparticles with a much larger surface area available for forming direct contacts between interacting surfaces.

Camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP) is a joint disease caused by a lack of lubricin, resulting in failed lubrication and abnormal deposition at the cartilage surface. Injection of recombinant lubricin (R-LUB) is a promising approach to improve the symptoms of the disease. However, the antifouling and lubrication properties of R-LUB on cartilage surfaces have not yet been studied. Here, the adsorption and lubrication behavior of a type II collagen (COL II) mimicking the cartilage surface upon R-LUB injection was followed by surface plasmon resonance spectroscopy and surface forces apparatus. The results indicated R-LUB can bind well on a COL II surface and COL II/R-LUB complex layer exhibited ultralow nonspecific adsorption of BSA (3.25 ng/cm 2 ) and LYS (0.26 ng/cm 2 ) compared to those of the COL II layer (32.7 ng/cm 2 , 7.26 ng/cm 2 ). Normal force measurements indicated there was always a repulsive force between COL II/R-LUB complex and different surfaces with -COO - , -NH 3 +, and -CH 3 groups. Likewise, COL II had a high coefficient of friction (∼0.48) with surface damage at 2 µm/s and a wear pressure of 1.56 MPa, while that of COL II/R-LUB complex was down to ∼0.008-0.13 with surface damage at 13 µm/s and a wear pressure of 11.96 MPa, which was 7.7 times higher than for COL II. Hence, R-LUB may act as an anti-adhesive and lubrication layer adsorbed on COL II surfaces to prevent direct contact. Our findings provide fundamental insights into the adsorption and lubrication behavior for understanding biological lubrication, especially the potential supplementation of R-LUB for treating CACP disease., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Protein-based underwater adhesives of marine organisms exhibit extraordinary binding strength in high salinity based on utilizing a variety of molecular interaction mechanisms. These include acid-base interactions, bidentate bindings or complex hydrogen bonding interactions, and electrochemical manipulation of interfacial bonding. In this Perspective, we briefly review recent progress in the field, and we discuss how interfacial electrochemistry can vary interfacial forces by concerted tuning of surface charging, hydration forces, and tuning of the interfacial ion concentration. We further discuss open questions, controversial findings, and new paths into understanding and utilizing redox-proteins and derived polymers for enhancing underwater adhesion in a complex salt environment.

After almost 35 years of truly successful and transformative advancements, Atomic Force Microscopy (AFM) and, in general, scanning probe microscopy still have a fundamental limitation. This is constant drift and uncontrolled motion of probe and tested surface structures with respect to each other. This is inherently linked to the currently accepted design principle-only forces are measured, and distances are inferred from force measurements and piezo motions. Here, we demonstrate and test a new setup, which combines advantages of AFM and the surface forces apparatus, where absolute distances are measured by Multiple Beam White Light Interferometry (MBI). The novel and unique aspect of this apparatus consists of a synergistic combination of white light interferometric measurement of the absolute distance by direct reflection from an AFM cantilever and a fast distance clamping and drift correction using an IR-laser Fabry-Pérot interferometry-based approach (FPI). We demonstrate the capabilities of the system by force/distance measurements, benchmarking of distance control by direct comparison of MBI and FPI, and discuss potential applications of the system. This novel setup has the potential to form, monitor, and stress a single molecule or ligand/receptor bond on the molecular hook with sub-nanometer control of molecular distances over in principle infinite times.

To reduce the global emissions of CO2, the aviation industry largely relies on new light weight materials, which require multifunctional coatings. Graphene and its derivatives are particularly promising for combining light weight applications with functional coatings. Although they have proven to have outstanding properties, graphene and its precursor graphene oxide (GO) remain far from application at the industrial scale since a comprehensive protocol for mass production is still lacking. In this work, we develop and systematically describe a sustainable up-scaling process for the production of GO based on a three-step electrochemical exfoliation method. Surface characterization techniques (XRD, XPS and Raman) allow the understanding of the fast exfoliation rates obtained, and of high conductivities that are up to four orders of magnitude higher compared to GO produced via the commonly used modified Hummers method. Furthermore, we show that a newly developed mild thermal reduction at 250 °C is sufficient to increase conductivity by another order of magnitude, while limiting energy requirements. The proposed GO powder protocol suggests an up-scaling linear relation between the amount of educt surface and volume of electrolyte. This may support the mass production of GO-based coatings for the aviation industry, and address challenges such as low weight, fire, de-icing and lightning strike protection.

Mineral nanoparticle suspensions with consolidating properties have been successfully applied in the restoration of weathered architectural surfaces. However, the design of these consolidants is usually stone-specific and based on trial and error, which prevents their robust operation for a wide range of highly heterogeneous monumental stone materials. In this work, we develop a facile and versatile method to systematically study the consolidating mechanisms in action using a surface forces apparatus (SFA) with real-time force sensing and an X-ray surface forces apparatus (X-SFA). We directly assess the mechanical tensile strength of nanosilica-treated single mineral contacts and show a sharp increase in their cohesion. The smallest used nanoparticles provide an order of magnitude stronger contacts. We further resolve the microstructures and forces acting during evaporation-driven, capillary-force-induced nanoparticle aggregation processes, highlighting the importance of the interactions between the nanoparticles and the confining mineral walls. Our novel SFA-based approach offers insight into nano- and microscale mechanisms of consolidating silica treatments, and it can aid the design of nanomaterials used in stone consolidation.

Phosphate- or chromate-based industrially produced conversion layers, while effectively increasing adhesion for organic coatings and corrosion resistance, come at the cost of environmentally problematic and harmful treatment solutions and waste. In this respect, layered double hydroxide (LDH)-based conversion layers offer an environmentally benign alternative without toxicologically concerning compounds in the treatment solution. Here, we study an LDH conversion layer on Zn-Al-Mg-coated steel (ZM-coated steel), which was produced by immersion into a carbonate- and magnesium-containing alkaline solution. The mechanism and kinetics of the conversion layer formation were investigated with in situ open circuit potential measurements, cyclic voltammetry (CV), and scanning electron microscopy (SEM). Acceleration of the LDH layer formation through high convection in the treatment solution was found. This was attributed to a higher oxygen availability at the metal/solution interface because no diffusion-limited state during the layer formation is reached due to high convection. The importance of oxygen within the kinetics indicates a corrosion-like mechanism, with cathodic and anodic sites on the steel sample. The LDH formation happens by co-precipitation of ions present in the treatment solution and dissolved ions from the ZM-coated steel. With CV, SEM, and X-ray diffraction, the growth of the LDH conversion layer was investigated with respect to the immersion time. It was found that after 30 s, the sample surface was almost fully covered with an LDH layer, and with the increasing immersion time, the layer grows in thickness. Increased understanding on the kinetics and mechanism of the LDH conversion layer formation on ZM-coated steel gives rise to a targeted optimization of the treatment solution and process parameters.

Catechol reaction mechanisms form the basis of marine mussel adhesion, allowing for bond formation and cross-linking in wet saline environments. To mimic mussel foot adhesion and develop new bioadhesive underwater glues, it is essential to understand and learn to control their redox activity as well as their chemical reactivity. Here, we study the electrochemical characteristics of functionalized catechols to further understand their reaction mechanisms and find a stable and controllable molecule that we subsequently integrate into a polymer to form a highly adhesive hydrogel. Contradictory to previous hypotheses, 3,4-dihydroxy-L-phenylalanine is shown to follow a Schiff-base reaction whereas dopamine shows an intramolecular ring formation. Dihydrocaffeic acid proved to be stable and was substituted onto a poly(allylamine) backbone and electrochemically cross-linked to form an adhesive hydrogel that was tested using a surface forces apparatus. The hydrogel's compression and dehydration dependent adhesive strength have proven to be higher than in mussel foot proteins (mfp-3 and mfp-5). Controlling catechol reaction mechanisms and integrating them into stable electrochemically depositable macroscopic structures is an important step in designing new biological coatings and underwater and biomedical adhesives.

Lignin has been spotlighted as an abundant renewable bioresource for use in material technologies and applications such as biofuels, binders, composites, and nanomaterials for drug delivery. However, owing to its complex and irregular structure, it is difficult to investigate its fundamental interaction mechanism, which is necessary to promote its use. In this study, a surface forces apparatus (SFA) was used to investigate the pH-dependent molecular interactions between a lignin nanofilm and five functionalized self-assembled monolayers (SAMs). The lignin nanofilm adhered most strongly to the amine-functionalized SAM, indicating that the molecular interactions with lignin were mainly electrostatic and cation-π interactions. The force-distance profile between lignin and a methyl-functionalized SAM revealed pH-dependent interactions similar to those between two lignin nanofilms. This finding indicates that the dominant cohesion mechanism is hydrophobic interactions. A quartz crystal microbalance with dissipation was used to investigate the adsorption of free lignin molecules on functionalized SAMs. Lignin molecules, which were free in solution, were most effectively adsorbed to the phenyl-functionalized SAM. To investigate whether the nanoscopic interaction forces could be extended to macroscopic properties, the compressive strength of activated carbon-lignin composites prepared at different pH values was evaluated. As the pH increased, the compressive strength decreased owing to the reduced hydrophobic interactions between the activated carbon and lignin, consistent with the SFA results. These quantitative results regarding lignin interactions can advance the potential use of lignin as an eco-friendly biomaterial.

Osteoarthritis is a prevalent degenerative joint disease characterized by progressive articular cartilage loss and destruction. The resultant increase in friction causes severe pain. The collagen I matrix (COL I) has been used clinically for cartilage repair; however, how COL I acts at cartilage surfaces is unclear. Here, we studied adsorption and lubrication of synovial fluid components, albumin, γ-globulin, and the phospholipid DPPC, on COL I under physiological conditions using surface plasmon resonance and an in situ sensing surface force apparatus. Our results revealed COL I had poor lubrication ability, a fairly high coefficient of friction (COF, μ = 0.651 ± 0.013), and surface damage under a 7 mN load. DPPC formed an improved lubricating layer on COL I (μ = 0.072 ± 0.016). In sharp contrast, albumin and γ-globulin exhibited poor lubrication with an order of magnitude higher COF but still provided benefits by protecting COL I from wear. Hence, DPPC on COL I may help optimize COL I implantation design.

Function and properties at biologic as well as technological interfaces are controlled by a complex and concerted competition of specific and unspecific binding with ions and water in the electrolyte. It is not possible to date to directly estimate by experiment the interfacial binding energies of involved species in a consistent approach, thus limiting our understanding of how interactions in complex (physiologic) media are moderated. Here, we employ a model system utilizing polymers with end grafted amines interacting with a negatively charged mica surface. We measure interaction forces as a function of the molecule density and ion concentration in NaCl solutions. The measured adhesion decreases by about 90%, from 0.01 to 1 M electrolyte concentration. We further demonstrate by molecular resolution imaging how ions increasingly populate the binding surface at elevated concentrations, and are effectively competing with the functional group for a binding site. We demonstrate that a competing Langmuir isotherm model can describe this concentration-dependent competition. Further, based on this model we can quantitatively estimate ion binding energies, as well as binding energy relationships at a complex solid|liquid interface. Our approach enables the extraction of thermodynamic interaction energies and kinetic parameters of ionic species during monolayer level interactions at a solid|liquid interface, which to-date is impossible with other techniques., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Lipid bilayer interactions are essential to a vast range of biological functions, such as intracellular transport mechanisms. Surface charging mediated by concentration dependent ion adsorption and desorption on lipid headgroups alters electric double layers as well as van der Waals and steric hydration forces of interacting bilayers. Here, we directly measure bilayer interactions during charge modulation in a symmetrically polarized electrochemical three-mirror interferometer surface forces apparatus. We quantify polarization and concentration dependent hydration and electric double layer forces due to cation adsorption/desorption. Our results demonstrate that exponential hydration layer interactions effectively describe surface potential dependent surface forces due to cation adsorption at high salt concentrations. Hence, electric double layers of lipid bilayers are exclusively dominated by inner Helmholtz charge regulation under physiological conditions. These results are important for rationalizing bilayer behavior under physiological conditions, where charge and concentration modulation may act as biological triggers for function and signaling.

Polymer brushes are widely used to alter the properties of interfaces. In particular, poly(ethylene glycol) (PEG) and similar polymers can make surfaces inert toward biomolecular adsorption. Neutral hydrophilic brushes are normally considered to have static properties at a given temperature. As an example, PEG is not responsive to pH or ionic strength. Here we show that, by simply introducing a polymeric acid such as poly(methacrylic acid) (PMAA), the highly hydrated brush barrier can change its properties entirely. This is caused by multivalent hydrogen bonds in an extremely pH-sensitive process. Remarkably, it is sufficient to reduce the pH to 5 for complexation to occur at the interface, which is two units higher than in the corresponding bulk systems. Below this critical pH, PMAA starts to bind to PEG in large amounts (comparable to the PEG amount), causing the brush to gradually compact and dehydrate. The brush also undergoes major rheology changes, from viscoelastic to rigid. Furthermore, the protein repelling ability of PEG is lost after reaching a threshold in the amount of PMAA bound. The changes in brush properties are tunable and become more pronounced when more PMAA is bound. The initial brush state is fully recovered when releasing PMAA by returning to physiological pH. Our findings are relevant for many applications involving functional interfaces, such as capture-release of biomolecules.

Degradation and dissolution of transparent semiconducting oxides is central to various areas, including design of catalysts and catalysis conditions, as well as passivation of metal surfaces. In particular, photocorrosion can be significant and plays a central role during photoelectrochemical activity of transparent semiconducting oxides. Here, we utilize an electrochemical flow cell combined with an inductively coupled plasma mass spectrometer (ICP-MS) to enable the in situ study of the time-resolved release of zinc into solution under simultaneous radiation of UV-light. With this system we study the dissolution of zinc oxide single crystals with (0001) and (101̅0) orientations. At acidic and alkaline pH, we characterized potential dependent dissolution rates into both the oxygen and the hydrogen evolving conditions. A significant influence of the UV radiation and the pH of the electrolyte was observed. The observed dissolution behavior agrees well with the surface chemistry and stabilization mechanism of ZnO surfaces. In particular, polar ZnO(0001) shows ideal stability at low potentials and under hydrogen evolution conditions. Whereas ZnO(101̅0) sustains higher dissolution rates, while it is inactive for water splitting. Our data demonstrates that surface design and fundamental understanding of surface chemistry provides an effective path to rendering electroactive surfaces stable under operating conditions.

The cellular membrane is central to the development of single-and multicellular life, as it separates the delicate cellular interior from the hostile environment. It exerts tight control over entry and exit of substances, is responsible for signaling with other cells in multicellular organisms and prevents pathogens from entering the cell. In the case of bacteria and viruses, the cellular membrane also hosts the proteins enabling invasion of the host organism. In a very real sense therefore, the cellular membrane is central to all life. The study of the cell membrane and membrane proteins in particular has therefore attracted significant attention. Due to the enormous variety of tasks performed by the membrane, it is a highly complex and challenging structure to study. Ideally, membrane components would be studied in isolation from this environment, but unlike water soluble proteins, the amphiphilic environment provided by the cellular membrane is key to the structure and function of the cell membrane. Therefore, model membranes have been developed to provide an environment in which a membrane protein can be studied. This review presents a set of tools that enable the comprehensive characterization of membrane proteins: electrochemical tools, surface plasmon resonance, neutron scattering, the surface forces apparatus and atomic force microscopy are discussed, with a particular focus on experimental technique and data evaluation., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Electrodeposition of metals is relevant to much of materials research including catalysis, batteries, antifouling, and anticorrosion coatings. The sacrificial characteristics of zinc used as a protection for ferrous substrates is a central corrosion protection strategy used in automotive, aviation, and DIY industries. Zinc layers are often used for protection by application to a base metal in a hot dip galvanizing step; however, there is a significant interest in less energy and material intense electroplating strategies for zinc. At present, large-scale electroplating is mostly done from acidic zinc solutions, which contain potentially toxic and harmful additives. Alkaline electroplating of zinc offers a route to using environment-friendly green additives. Within the scope of this study an electrolyte containing soluble zinc hydroxide compound and a polyquarternium polymer as additive were studied during zinc deposition on gold model surfaces. Cyclic voltammetry experiments and in-situ electrochemical quartz crystal microbalance with dissipation (QCM-D) measurements were combined to provide a detailed understanding of fundamental steps that occur during polymer-mediated alkaline zinc electroplating. Data indicate that a zincate-loaded polymer can adsorb within the inner sphere of the electric double layer, which lowers the electrostatic penalty of the zincate approach to a negatively charged surface. X-ray photoelectron spectroscopy also supports the assertion that the zincate-loaded polymer is brought tightly to the surface. We also find an initial polymer depletion followed by an active deposition moderation via control of the zincate diffusion through the adsorbed polymer.

The molecular-scale structure and dynamics of confined liquids has increasingly gained relevance for applications in nanotechnology. Thus, a detailed knowledge of the structure of confined liquids on molecular length scales is of great interest for fundamental and applied sciences. To study confined structures under dynamic conditions, we constructed an in situ X-ray surface forces apparatus (X-SFA). This novel device can create a precisely controlled slit-pore confinement down to dimensions on the 10 nm scale by using a cylinder-on-flat geometry for the first time. Complementary structural information can be obtained by simultaneous force measurements and X-ray scattering experiments. The in-plane structure of liquids parallel to the slit pore and density profiles perpendicular to the confining interfaces are studied by X-ray scattering and reflectivity. The normal load between the opposing interfaces can be modulated to study the structural dynamics of confined liquids. The confinement gap distance is tracked simultaneously with nanometer precision by analyzing optical interference fringes of equal chromatic order. Relaxation processes can be studied by driving the system out of equilibrium by shear stress or compression/decompression cycles of the slit pore. The capability of the new device is demonstrated on the liquid crystal 4'-octyl-4-cyano-biphenyl (8CB) in its smectic A (SmA) mesophase. Its molecular-scale structure and orientation confined in 100 nm to 1.7 μm slit pores was studied under static and dynamic nonequilibrium conditions.

Understanding interaction force versus distance profiles of supported lipid bilayers (SLBs) is relevant to a number of areas, which rely on these model systems, including, e.g., characterization of ligand/receptor interactions or bacterial adhesion. Here, the stability of 4 different SLB architectures was compared using the surface forces apparatus (SFA) and atomic force microscopy (AFM). Specifically, the outer envelope of the bilayer systems remained constant as 1,2-dipalmitoyl- sn -glycero-3-phosphocholine (DPPC). The inner layer was varied between DPPC and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) both on mica, and self-assembled monolayers (SAMs) of hexadecanethiol and the polymer-tethered diphytanylglycerol-tetraethylene glycol-lipoid acid (DPhyTL) on smooth gold surfaces. In that same order these gave an increasing strength of interaction between the inner layer and the supporting substrate and hence improved stability under highly adhesive conditions. Detachment profiles from highly charged and highly adhesive contacts were characterized, and approach characteristics were fitted to DLVO models. We find increasing stability under highly adhesive loads, approaching the hydrophobic limit of the adhesive energy between the inner and outer layers for the SAM-based systems. For all four SLBs we further compare AFM surface topographies, which strongly depend on preparation conditions, and the DLVO fitting of the SFA approach curves finds a strong charge regulation behavior during interaction, dependent on the particular model system. In addition, we find undulation characteristics during approach and separation. The increased stability of the complex architectures on a gold support makes these model systems an ideal starting point for studying more complex strongly adhesive/interacting systems, including, for example, ligand/receptor interactions, biosensing interactions, or cell/surface interactions.

The interfacial decomposition products forming the so-called solid-electrolyte interphase (SEI) significantly determine the destiny of a Li-ion battery. Ultimate knowledge of its detailed behavior and better control are required for higher rates, longer life-time, and increased safety. Employing an electrochemical surface force apparatus, it is possible to control the growth and to investigate the mechanical properties of an SEI in a lithium-ion battery environment. This new approach is here introduced on a gold model system and reveals a compressible film at all stages of SEI growth. The demonstrated methodology provides a unique tool for analyzing electrochemical battery interfaces, in particular in view of alternative electrolyte formulations and artificial interfaces., Competing Interests: The authors declare no conflict of interest.

Multiple beam interferometry (MBI) evolved as a powerful tool for the simultaneous evaluation of thin film thicknesses and refractive indices in Surface Forces Apparatus (SFA) measurements. However, analysis has relied on simplifications for providing fast or simplified analysis of recorded interference spectra. Here, we describe the implementation of new optics and a generalized fitting approach to 4 × 4 transfer matrix method simulations for the SFA. Layers are described by dispersive complex refractive indices, thicknesses, and Euler angles that can be fitted, providing modeling for birefringent or colored layers. Normalization of data by incident light intensities is essential for the implementation of a fitting approach. Therefore, a modular optical system is described that can be retrofit to any existing SFA setup. Real-time normalization of spectra by white light is realized, alignment procedures are considerably simplified, and direct switching between transmission and reflection modes is possible. A numerical approach is introduced for constructing transfer matrices for birefringent materials. Full fitting of data to the simulation is implemented for arbitrary multilayered stacks used in SFA. This enables self-consistent fitting of mirror thicknesses, birefringence, and relative rotation of anisotropic layers (e.g., mica), evaluation of reflection and transmission mode spectra, and simultaneous fitting of thicknesses and refractive indices of media confined between two surfaces. In addition, a fast full spectral fitting method is implemented for providing a possible real-time analysis with up to 30 fps. We measure and analyze refractive indices of confined cyclohexane, the thickness of lipid bilayers, the thickness of metal layers, the relative rotation of birefringent materials, contact widths, as well as simultaneous fitting of both reflection and transmission mode spectra of typical interferometers. Our analyses suggest a number of best practices for conducting SFA and open MBI in an SFA for increasingly complex systems, including metamaterials, multilayered anisotropic layers, and chiral layers.

The surface density of charged sulfonic acid head groups in a perfluorosulfonic acid (PFSA) proton exchange membrane determines the hydrophilicity of the ionic channels and is thus critical for the structuring and transport of water and protons. The mechanism through which the head group density affects the structuring of water and ions is unknown, largely due to experimental challenges in systematically varying the density in an appropriate model system resembling the ionic channels. Here, we present a model system for PFSA membrane ionic channels using self-assembled monolayers with a tunable surface density of sulfonic acid and methyl groups to tune surface hydrophilicity. Atomic force microscopy force-distance measurements were used to quantify the hydration forces and deduce the interfacial electrolyte structure. The measured force profiles indicate a pronounced change of the electrolyte layering density at the surface with an unexpectedly sharp hydrophobic-to-hydrophilic transition when the surface shows a contact angle of ∼37°. Using an extended Derjaguin-Landau-Verwey-Overbeek model including the Hydra force, we quantify diffuse double layer charges and characteristic hydration lengths as a function of sulfonic acid group density on the surface. Translating our results to PFSA membranes, these findings have two implications: (1) the density of sulfonic acid head groups can have a dramatic effect on the local solvent structuring of water inside the ionic channels and (2) they support a view where two types of water (solution) exist in PFSA ionic channels - a structured (shell/surface) and a non-structured (bulk) water. This offers an interesting perspective on how different head group densities lead to changes in water and proton transport and macroscopic membrane conductivity properties based on hydration layer characteristics.

Understanding of electrical double layers is essential to all electrochemical devices, particularly at high charge carrier concentrations. Using a combined approach (surface force apparatus, zeta potential, infrared spectroscopy), we propose a model for the interfacial structure of triglyme electrolytes on muscovite mica. In contact with the pure triglyme, a brush-like polymeric structure grows on the mica surface. When lithium triflate is present in the triglyme, this structure is suppressed by anion adsorption and an extended double layer is formed. A surprising result of great fundamental significance is that the effective screening length measured by surface force apparatus at considerable lithium triflate concentrations (above 0.2 M) is substantially higher than expected from the Debye-Hückel theory. This suggests a high degree of complex salt association as a novel characteristic feature of salt-containing electrolytes.

The capabilities of atomic force microscopes and optical tweezers to probe unfolding or surface-to-molecule bond rupture at a single-molecular level are widely appreciated. These measurements are typically carried out unidirectionally under nonequilibrium conditions. Jarzynski's equality has proven useful to relate the work obtained along these nonequilibrium trajectories to the underlying free energy of the unfolding or unbinding process. Here, we quantify biases that arise from the molecular design of the bond rupture experiment for probing surface-to-molecule bonds. In particular, we probe the well-studied amine/gold bond as a function of the linker's length which is used to anchor the specific amine functionality during a single molecule unbinding experiment. With increasing linker length, we observe a significant increase in the average work spent on polymer stretching and a strongly biased estimated interaction free energy. Our data demonstrate that free energy estimates converge well for linker lengths below 20 nm, where the bias is <10-15%. With longer linkers severe methodical limits of the method are reached, and convergence within a reasonable number of realizations of the bond rupture is not feasible. Our results also provide new insights into stability and work dissipation mechanisms at adhesive interfaces at the single-molecular level, and offer important design and analysis aspects for single-molecular surface-to-molecule experiments.